The present invention relates to a polishing apparatus for polishing thin work pieces, e.g., wafers.
A conventional polishing apparatus is disclosed in Japanese Patent Gazette No. 11-48126. The polishing apparatus is shown in FIG. 7. In the polishing apparatus 10, an upper polishing plate 12 is vertically moved by a cylinder unit 14 provided on a base 13 together with a rod 14a, a connecting section 14b, a rotary plate 15 and connecting rods 16. The connecting section 14 includes a suitable mechanism, e.g., universal joint, for rotating the upper polishing plate 12 along an upper face of a lower polishing plate 18. Keys 19 are engaged with key grooves of a rotor head 20. With this structure, the upper polishing plate 12 is rotated in a horizontal plane by a motor with a shaft 20a, gears and a reduction gear unit 21. On the other hand, the lower polishing plate 18 is rotated by the motor 22 with a hollow shaft 18a, gears and the reduction gear unit 21. A sun gear 23 is rotated by the motor 22 with a shaft 23a, gears and the reduction gear unit 21. An internal gear 24 is also rotated by the motor 22 with a shaft 24a, gears and the reduction gear unit 21. Carriers 25 are engaged with the sun gear 23 and the internal gear 24, so that the carriers 25 perform sun-and-planet motion. A work piece 26 is fitted in a through-hole of each carrier 25 and moved together with the carrier 25. By rotating the polishing plates 12 and 18, both surfaces of each work piece 26 can be polished.
In the apparatus 10, an upper polishing plate 12 is suspended by a cylinder unit 14 while polishing work pieces 26. A pressing force of the upper polishing plate 12, which is applied to the work pieces 26, is adjusted by adjusting pressure of a pressurized fluid, which is supplied to a lower cylinder chamber of the cylinder unit 14. By adjusting the pressure, the pressing force from the upper polishing plate 12 to the work pieces 26 can be adjusted in a range, which is from a force smaller than weight of the upper polishing plate 12 to a force equal to the weight of the upper polishing plate 12. Note that, the pressing force greater than the weight of the upper polishing plate 12 can be applied to the work pieces 26 by supplying the pressurized fluid to an upper cylinder chamber of the cylinder unit 14.
However, the conventional polishing apparatus 10 has following disadvantages.
Firstly, a piston, which is provided to an upper end of a rod 14a of the cylinder unit 14, slides on an inner face of a cylinder of the cylinder unit 14. Therefore, friction is generated between a seal member of the piston and the inner face of the cylinder, so that it is difficult to precisely control the vertical movement of the upper polishing plate 12. Especially, when the cylinder unit 14 is actuated from a stopping state, friction coefficient is changed from a high static frictional coefficient to a low dynamic frictional coefficient, so that knocking easily occurs.
Driving forces for driving the polishing plates 12 and 18, the sun gear 23 and the internal gear 24 are transmitted by various gears, and the keys 19 are engaged with the key grooves of the rotor head 20 connected to the upper polishing plate 12. Therefore, backlashes are occurred between gears and between keys 19 and the key grooves, so that vibrations easily occur therebetween. If the work pieces 26 are thin, they will be damaged or broken by the vibrations.